mpm_event, mpm_worker: Fix computation of MinSpareThreads' lower bound

according the number of listeners buckets.

Submitted by: ylavic
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1748336 13f79535-47bb-0310-9956-ffa450edef68